Field of the Invention
The present invention is related to epoxy thermosets. More particularly, the present invention is related to tougheners for epoxy thermosets.
Introduction
Epoxy thermosets are inherently brittle due to their highly cross-linked polymer network. Such a drawback has resulted in limited use of epoxy resins in many demanding applications where toughness is required. In recent years, new developments in composites, coatings, and electronics require epoxy thermosets with greater thermal stability. Increasing the thermal stability of the epoxy polymer network often requires further tightening of the polymer network with increased crosslink density, resulting in a much more brittle polymer network.
Among the methods for solving the problems, it has been attempted to blend a rubbery ingredient with an epoxy resin. Examples of these methods include (1) heating partially cross linked rubbery random copolymer particles prepared by emulsion polymerization using a nonionic emulsifier or the like to a temperature higher than the cloud point of the emulsifier, thereby coagulating them, then optionally washing the coagulate with water and mixing the same with an epoxy resin, (2) mixing a rubbery polymer latex and an epoxy resin and then evaporating off the water content to obtain a mixture, and (3) mixing a rubbery polymer latex with an epoxy resin under the presence of an organic solvent to obtain a mixture.
The methods of (1) and (2) described above are methods of dispersing polymer particles that are adhered to each other by coagulation in a viscous epoxy resin. Since the rubbery polymer particles are physically bonded to each other, pulverization or a re-dispersion operation with a considerably large mechanical shearing force is required upon mixing with the epoxy resin. The higher viscosities of the epoxy resins often make it more difficult to uniformly re-disperse the rubbery polymer particles. As a result, such processes may leave an unmixed portion, and lumpy agglomerations are sometimes formed in the unmixed portion. Furthermore, addition of polymer particles to the viscous epoxy resin often leads to a further increase in viscosity, resulting in difficulties using the dispersions. Use of epoxy reactive diluents in place of liquid epoxy resins can significantly reduce viscosity of the dispersions, but it is usually accompanied by sacrificing other properties, such as thermal stability, mechanical strength and chemical resistance.
Method (3) described above does not include a coagulating operation, so an epoxy resin composition with rubbery polymer particles dispersed uniformly can be obtained, but a great amount of water content present together with the organic solvent in the system must be separated or evaporated off. Separation of the organic solvent layer and the aqueous layer can require as long one day and one night. Additionally, the organic solvent layer and the aqueous layer are difficult to separate substantially since they form a stable emulsified suspension. Further, in a case of removing the water content by evaporation, a great amount of energy is necessary and, in addition, water soluble impurities such as an emulsifier or sub-starting materials usually used in the production of rubbery polymer latexes remain in the composition to degrade the quality.
Therefore, a need exists for a toughening system with low viscosity that provides a uniform distribution of rubbery particles in an epoxy thermoset matrix.